Such a hair treatment device is, e.g., known from the international patent application published as WO 2010/106480 A1. This patent application describes a device for imaging a hair near a skin surface of a body part. The device comprises a light source for generating a light beam with an incident polarization and a detector for detecting radiation returning from said hair. The detector has separate photodiodes for detecting light with the incident polarization and light with a polarization direction orthogonal to the incident polarization. The ratio of light intensities detected by the separate photodiodes is a probability measure for the presence of a hair at the tested skin location.
It has turned out that the sensitivity and specificity of this known detector is largely dependent on the angle between the polarization orientation of the incident light and the orientation of the hair to be detected and on the focusing depth inside the hair. Because the orientation of the hairs and the focusing depth inside the hair differs from hair to hair and over time, the hair-skin contrast obtained by the known detector is not entirely satisfactory.
Furthermore, the orientation of the hair in the plane perpendicular to the direction of light is a critical parameter that determines the sensitivity and specificity of hair detection. This in turn depends on the numerical aperture of the focusing element. With NA=0.9, the maximum detectable angle in the axial direction is limited to 30° whereas with NA=0.3, the maximum detectable is only 10°. In short, the hair is detected with optimal contrast when the illumination beam is focused at the desired depth inside the hair when it is brought within the detection cone.